In aerobic composting, oxygen is needed for decomposition. Decomposing organisms use up initial air supplies quickly. Without sufficient oxygen to fuel the composting organisms, the process slows. Decomposition won't screech to a complete halt, but it will definitely slacken. Turning the pile periodically to add more oxygen kicks it back into gear.
If you don't want to turn your pile frequently (or at all), don't worry. Compost will still make itself, it'll just take longer. However, it'll help if you incorporate one or more of the following methods to promote aeration as you construct your pile:
Start with a layer of larger, woody branch trimmings, dead perennial stems, or cornstalks at the bottom of the pile (this lets in lots of air from the base), and scatter some throughout the pile as you build it. This makes it a bit more difficult to turn the pile because large, woody chunks get stuck in fork tines.
Lay a wooden shipping pallet down as the foundation of your compost pile. (If you garden where soil drainage is poor, do this in addition to incorporating trimmings as noted in the preceding bullet; otherwise, one or the other is sufficient.) The pallet sits a few inches aboveground, allowing air circulation beneath. Try this if achieving sufficient aeration is a problem because the ground remains damp or it rains a lot.
Insert one or more airflow tubes into the midst of the pile during construction, adding material around them as you build. Make tubes from leftover lengths of PVC (polyvinyl chloride) drainpipe (any diameter over 2 inches [5 centimeters] is good), chicken wire, or hardware cloth. Drill holes into PVC pipe every 6 inches (15 centimeters) or so along the length of the pipe. Or, roll chicken wire or hardware cloth into a cylindrical shape. Tubes should be long enough to reach the bottom of the pile and extend to the top of the bin. Airflow tubes serve double-duty because they can also be used to add water to the pile's interior if it dries out.
Airflow tubes allow airflow through the pile to aid the decomposition process.